1. Field of the Invention
The present invention relates to an f.theta. lens system built in a laser scanning unit employing a rotary polygon mirror.
2. Description of the Related Art
A laser scanning unit for scanning image data onto a photoconductive scanned face is widely used as a primary component in a laser beam printer, facsimile, digital copier, etc. The term, laser scanning unit, will be referred to as "LSU" throughout this specification.
FIG. 1A simply shows the conceptual structure of a LSU. To scan image data from an image processor onto a photoconductive drum 3, the LSU employs a laser source 1 and a rotary polygon mirror 2. Generally, a semiconductor laser diode is used as the laser source 1 and the photoconductive materials are coated on the surface of a cylindrical drum to form a photoconductive drum 3.
Polygon mirror 2 is usually forms a square, hexagon, or an octagon, of which the reflecting faces must be polished into extremely high flatness to reduce image deterioration.
Being turned on or off according to binary levels of image data, the laser source 1 emanates a laser beam onto the reflecting face of the polygon mirror 2. Since the polygon mirror 2 rotates, the laser beam is deflected from the reflecting face and is thereafter line scanned in a dot shape onto the photoconductive drum 3. The laser beam changes its scanning line along the respective reflecting faces of the polygon mirror. This can be done by rotating the photoconductive drum 3 at a regular speed. Like the above, scanning a laser beam in the axial direction of a photoconductive drum by rotating a polygon mirror is called "main scanning"; changing scanning lines in the direction perpendicular to the main scan by rotating the photoconductive drum is called "cross scanning".
However, a problem arises in such a structure: Since the distance between the central part of a scanned face (i.e., the photoconductive drum) and the reflecting face differs from that between the end parts of the scanned face and the reflecting face, the laser beam cannot be uniformly focused along the entire scanned face. This causes severe damage to image replica appliances for which the picture quality is a most important factor.
To avoid the above problem, several correction lenses 4 and 5 are placed between the polygon mirror 2 and the photoconductive drum 3, as depicted in FIG. 1B. This correction lens is often called an f.theta. lens, expressing mathematically its focal length correcting function. The f.theta. lenses 4 and 5 must correct, as shown in FIGS. 1B and 1C, beam spots not only in the main scan direction but also in the cross scan direction, to perform complete dot correction (correction into an ellipse whose axis in the cross scan direction is slightly longer than that in the main scan direction).
Thus the f.theta. lenses 4 and 5 must be designed such that each of the refractive faces has a given radius of curvature in the cross scan direction as well as in the main scan direction. Of course, the two radius values have different values. Such a lens is called a toric face lens.
FIGS. 1B and 1C show a practical LSU structure, in which the conceptual LSU in FIG. 1A is completed by several ancillary components. A monochromatic laser beam (usually 780-nm wavelength) emitted from laser diode 1 is modulated into a parallel beam through a collimation lens 6, and the modulated laser beam is incident on the reflecting face of the polygon mirror 2. The beam is deflected by the reflecting face rotating at a high speed in conjunction with the polygon mirror 2 and is thereafter line scanned in the main scan direction onto the photoconductive drum 3. Because of this deflection function, a rotary polygon mirror is sometimes called "deflector". At this point, the f.theta. lenses 4 and 5 properly corrects the focal length differences in the beam to scan the beam uniformly onto the photoconductive drum 3 at an equal velocity and a regular linearity, thereby forming equal beam spots on the drum 3.
A beam detector 7 and a plane mirror 8 are provided to line up the starting and ending points of each scanning line at both scanned limits of the photoconductive drum 3. The plane mirror 8 is installed above one end of the photoconductive drum 3, facing the beam detector 7 with a given angle. This angle must be set to reflect the laser beam exactly towards the beam detector 7 when the laser beam is incident on the mirror 8. The reflected beam is then transformed into an electrical signal by the beam detector 7 and the related circuits (not shown), and is provided as a pulse signal to a controller (this pulse signal is usually called a synchronous signal).
In practice, the deflected laser beam is reflected on a narrow and long plane mirror 9 to be scanned onto the photoconductive drum 3, so that the mechanical distance from the polygon mirror 2 (i.e., the deflector) to the drum 3 (i.e., the scanned face) is reduced and the beam running direction is changed. The plane mirror 9 changes only the beam running direction, not beam characteristics. The reflecting angle of the mirror 9 can be modified according to designer discretion. By doing so, the mechanical dimension of the LSU, and thus an assembled final product, can be reduced, and the design job becomes easier.
U.S. Pat. No. 5,136,418 discloses one of the LSUs employing toric face lenses in a lens system which corrects a scanned image, using two f.theta. lenses. In the '418 patent, two of the four refractive faces, i.e., the front and rear of the two f.theta. lenses, are the toric faces.
In fabricating a lens, since it is quite difficult to process a toric face which has various radii of curvature in the every direction and the permissible tolerance range is narrow (typically 2/100-5/100 mm), this type of lens is not suitable for mass production. Thus the possibility of inferior quality LSUs in process, and therefore the total costs, increase.
Conventional LSUs commonly include a group of several f.theta. lenses having two or more toric faces. Because of many toric faces, the yield of the LSUs decreases and the costs increase, for the above reasons. In addition, since the refractive index of a lens must be relatively larger, high refractivity materials must be used for the lens.